Field of the Invention
This invention relates to a method and apparatus for on-line measurement of the permeation characteristics of a permeant through dense nonporous membrane and more particularly, to a method and apparatus for on-line measurement of the permeation characteristics (permeation rate, permeate composition, etc.) of a permeant in liquid, vapor and gas phases through dense polymeric membrane.
According to the xe2x80x9csolution-diffusion modelxe2x80x9d, a steady-state transport of a permeant through dense polymeric membrane is generally composed of the following three step mechanism:
Step 1: the sorption of permeating molecules into the membrane.
Step 2: diffusion through the membrane.
Step 3: desorption from the membrane.
On this occasion, the diffusion coefficient of the gas, vapor or liquid absorbed inside of the membrane is a function of the swelling state of the membrane because the molecules absorbed inside the membrane originate from a plasticization which affects the flexibility of polymer chains.
Namely, the diffusion coefficient varies with the membrane thickness, since the above-stated absorbed molecules cause the anisotropic swelling of the membrane in the direction from the top surface to the bottom surface.
The swelling state of the membrane and the solubility vary with the phases of permeants, which may be liquid, gas, or vapor, and the method for measuring permeation characteristics varies as well.
Since a gaseous permeant has small attraction and affinity for the membrane, it does not contribute much to the swelling state, and the permeation rate is not high due to the low solubility.
Therefore, for the gaseous permeants, the two methods based on the steady-state transport (J. Appl. Polym. Sci., 18 (1974) 351, J. Appl. Polym. Sci., 12 (1968) 2615) and the non-steady state transport (J. Appl. Polym. Sci., 26 (1957) 151, J. Appl. Polym. Sci., 14 (1970) 523) are used to determine the permeability and the diffusion coefficient.
For the evaluation of permeation characteristics such as permeability and the diffusion coefficient, the steady-state transport method known as time-lag technique is widely used. Moreover, in the steady state transport method, the permeation rate and the amount of permeation are determined indirectly or relatively by using mass spectrometer or by measuring the change of pressure in the bottom of the membrane. However, the steady state transport method does not directly measure the absolute permeation rate, and there are difficulties in analyzing permeants with high permeation rate or good affinity to a membrane.
The above nonsteady state transport method can be divided to sorption method and desorption method based on swelling experiments and the free-volume theory (Adv. Polym. Sci., 3 (1961) 1) is a key part thereof. The disadvantages of the nonsteady state transport method are that the process is complicate and the calculation results are very sensitive to the accuracy at the measurement.
Also, the diffusion coefficient obtained from the nonsteady state transport method is hard to apply to a steady state transport process, because the membrane has different adaptation hysteresis.
In addition, the above nonsteady state transport method has a disadvantage in the on-line measurement of the time-varying permeation characteristics.
For the vapor and/or liquid permeation, the vapor and/or liquid substance has generally large attraction with the membrane material and relatively high solubility, because when designing the membrane material, the polymer with good affinity for the vapor and liquid material is selected. Thus, new method different from the gaseous material should be chosen for the measurement of the permeation characteristics of the vapor and/or liquid material due to its high permeation rate.
So far, the solubility and the diffusion coefficient of the vapor and/or liquid permeants have been obtained by using the nonsteady state transport
However, as mentioned above, there is a problem that the adaptation hysteresis in an actual permeation process to the membrane is different from that in characteristic measurement process.
At present, the most prevalent method for measuring the permeation characteristics is a method to analyze the permeation rate and the permeant composition condensed for a finite time after passing through the membrane.
However, when the permeation rate is low, more time is required for the measurement so that the accuracy of measurement becomes poor in the analysis of the permeant after it is condensed.
The present invention is intended to solve disadvantages and problems occurred by the conventional methods for measuring the permeation characteristics of permeants. This invention is intended to facilitate various research activities, such as simultaneous analysis of diffusion coefficient, solubility coefficient, permeation rate, and permeant composition, and presentation of new interpretation about the permeation behavior, kinetics research, and so on.
The object of this invention is to provide a method and apparatus for on-line measurement of permeation characteristics of permeants through dense membrane, for example, permeation rate and permeation concentration accurately and reliably, not only for a steady state but also for a nonsteady state with time.